Push-pull amplifier circuit



Nov, 12, 1940. H. MAYER I PUSH-PULL AMPLIFIER CIRCUIT Filed June 8, 1957 INVENTOR HANS MAYER I ATTORNEY o with the plate circuits of the two tubes.

Patented Nov. 12, 1940 PATENT OFFICE PUSH-PULL AMPLIFIER omourr Hans Mayer, Berlin-Charlottenburg, Germany,

assignor to Siemens & Hals'ke Akt., Siemensstadt, near Berlin, Germany, a corporation of Germany Application June 8, 1937, Serial No. 146,996 In Germany June 10, 1936 3 Claims.

A type of push-pull amplifier is known in the art in which, with a view to causing the necessary phase opposition, a second tube is employed whose grid circuit is fed from the plate circuit of the first tube.

One of the objects of the present invention is to avoid certain disadvantages hereinafter pointed out of such prior art amplifiers.

The invention will be described in connection with the accompanying drawing, wherein Fig. 1 illustrates a known push-pull amplifier circuit, Figs. 2A and 2B illustrate circuits which will serve to explain the present invention, and Fig. 3 discloses a circuit which embodies the present invention.

Referring now to Fig. l, the circuit shown is that of a known push-pull amplifier utilizing first and second tubesvl and V2, respectively.

The grid circuit of thesecond tube V2 is impressed with a voltage which is. derived from across the resistance R included in the plate circuit of the first tube V1, while the signal input potential to be amplified E is fed to the input circuit of tube V1. The load resistance Z is normally connected by means of a transformer T Inasmuch as the voltage in the plate circuit is in phase opposition to the input potential, the two tubes will, at their input ends, be excited in phase opposition with the result that the arrangement operates like a normal or conventional pushpull amplifier, though with this advantage that a distinct input transformer having a tapped secondary wiriding becomes superfluous. The choke-coil CH which is inserted in the joint plate circuit may be used for the purpose of making the two plate circuits independent of each other for alternating current.

However, from the viewpoint of energy adaptation of the load resistance to the tubes, there are discovered certain disadvantages in this circuit scheme. While in a normal push-pull arrangement the resultant inner resistance, as is well known, is equal to 2R1 (where R1 is the inner resistance of a tube), the circuit shown in Fig. 1 behaves as if the inner resistance were not 2R1,

but rather where D is the reciprocal of 'the amplification constant of the tube and h is the part of the plate alternating voltage which is impressed upon the grid of the tube V2 in order that both tubes may be loaded and be operated at the same pointson the straight portion of the characteristic.

The quotient has the value 2, if the term h is so chosen that both tubes are worked to identical degrees. What then results is a resultant inner resistance of 4=Ri contradistinct from whatis obtained in the normal push-pull scheme where it is 2R1. When matched to the said raised inner resistance, the amplifier works no longer under conditions which are most favorable from the viewpoint of maximum power or'output, and this is undesirable.

Now, this shortcoming is obviated according to the present invention by that part. of the plate alternating potential of the first tube is impressed upon the very grid of the said tube. By this step, the resultant inner resistance may be reduced to the value 21%;, so that the circuit becomes equivalent to the conventional push-pull circuit also when looked at from the viewpoint of adaptability.

basic idea of the invention. The distinction in contrast to Fig. 1 is that thepotential E0 to be amplified is active no longer between the grid and the filament of the first tube, but rather between the grids of both tubes. Hence, in the grid circuit of the first tube V1 there is thus active not merely the signal or input potential E0 to be amplified, but in series therewith also the voltage which has been tapped across resistance R, while upon the grid of the tube V2 will act only this tapped voltage.

The following analysis will serve-to explain the improved operation afi'orded by the present Fig. 3 shows an exemplified embodiment of the invention. Fig. 2A shows a conventional pushpull amplifier. The voltage on one plate is as is well known, where ,u. the amplification constant of the tube, or l/D. Hence, if we wish to obtain symmetrical grid excitation for the lower tube V2 from the plate voltage of the upper tube V1 we must take ofi a fraction h of the plate voltage of the upper tube such that In particular, if the load Z of each tube is matched to its inner resistance R1 the value of h becomes 2.

The various currents flowing in the circuit of Fig. l are exactly the same as in Fig. 2A, which is the ordinary push-pull amplifier. However, when we examine the behaviour of the circuit of Fig. 1 when a voltage is impressed in the output circuit and not in the input a difference between Fig. l and Fig. 2A is discovered. Referring to Fig. 2B which is the same as Fig. 1 arranged to show a voltageE impressed in the load circuit, the resulting plate potentials being E1 and -E2, and the resulting current being 1, we may write three equations as follows:

. E1 that would be required to produce the current I if there were no grid potential.

' These three equations are readily solved to give -=2R.-+thR.-+Z

hence the apparent inner resistance of the amplifier is Ri( +,uh). If it is. now assumed that the load Z was chosen to match the tube impedance for the condition where the impressed voltage is on the input side of the amplifier, namely Z=2Ri as mentioned above, then the value of h required, 'as also mentioned above, is 2. Hence the effective tube resistance of Fig. 1 as measured from the output circuit and with h chosen suitably for operating with an exact impedance match between amplifier and load, is 4R1 which is twice as great as in the case of an ordinary push-pull amplifier. This increase in effective amplifier impedance measured from the output side reduces the damping action of the amplifier upon transient vibrations of a loudspeaker used as a load and hence is deleterious to faithful reproduction of sound.

A qualitative visualization of the difference in apparent resistance between Figs. 2A and 23 may be obtained by considering that when voltage is applied in the output of Fig. 2B, the resistance of the tubes is necessarily increased by the fact that some voltage is applied to the grid of the lower tube in a sense to oppose the flow of current therethrough while in Fig. 2A no voltage is impressed on either grid. Similarly the operation of circuit Fig. 3 may be explained by observing that when voltage is impressed in the output circuit, the resistance of the amplifier system is simultaneously increased by the impression of a certain amount of voltage on the grid of the lower tube V2 in a sense to oppose flow of current, and decreased by the impression of this same amount of voltage on the grid of the upper tube V1 in a sense to facilitate the fiow of cur- H rent. Since these two actions offset each other the resultant current is substantially the same as if neither grid were supplied with any voltage, hence the efiective internal impedance of the amplifier of Fig. 3 is the same as that of the ordinary push-pull amplifier of Fig. 2A, namely 2R1, but the advantage is retained of dispensing with a differential input transformer.

While it is true that with the circuit according to the invention there is a certain loss of gain which, when both tubes are operated at the same point of their characteristics, amounts to around 0.7 neper, this circumstance becomes negligible and immaterial due to the fact that in power or end stages the chief desideratum is a high undistorted power delivery rather than high gain. As to the rest, a scheme of the sort here disclosed results in a reduction of non-linear distortion.

What is claimed is:

1. An amplifier arranged for push-pull operation comprisinga load circuit, first and second tubes having their cathodes maintained at a common potential and their plates connected to opposite ends of said load circuit, means for impressing in the grid circuit of the second tube an alternating voltage equal to a fraction of the plate alternating voltage of the first tube, and

means for impressing in the grid circuit of thefirst tube a voltage equal to the sum of two voltages, one of which is said fraction of the plate alternating voltage of said first tube, and the other of which is the input voltage to be amplified.

2. An amplifier according to claim 1 wherein the tubes are similar to each other, and the fraction of the plate alternating voltage impressed in the grid circuit of the second tube is adjusted to a value approximately equal to twice the reciprocal of the amplification constant of the tubes.

3. An amplifier according to claim 1, wherein the two voltages impressed in the grid circuit of the first tube are connected in series between the grid and cathode of said tube.

' HANS MAYER. 

